Digital to analog converters (DACS) having precise component values are difficult to fabricate. A known method to provide a digital to analog converter with precise component values is to use a resistive DAC and to precisely trim the values of the resistors. This is commonly implemented by using thin-film resistors which are laser trimmed. Thin-film resistors are implemented by putting a thin film layer over a thick field oxide layer. However, laser trimming is generally restricted to thin film processing and must be implemented before packaging at a wafer probe stage. If the die is stressed during or after packaging, the value of the resistors may change. Another known method of generalized resistor trimming which does not require thin film processing for obtaining a more precise DAC is to use a resistor of a predetermined value coupled in parallel to a plurality of discrete resistors of substantially larger value by parallel links. Selected links are then disconnected to obtain a precise resistive value.
Yet another method of providing a precise DAC is to use a PROM for offset, linearity, full-scale calibration or temperature compensation as disclosed by Barry Harvey in an article entitled "A Monolithic 12b System DAC", in the IEEE International Solid-State Circuits Conference, February, 1983, pp. 182-183. A PROM is used to selectively couple currents to various internal nodes thereby correcting the DAC by injecting a compensating current to null output errors created in part by non-ideal device parameters. Existing D/A converters predominately apply compensation techniques in bipolar integrated circuit technologies.
Unfortunately, capacitors cannot be reliably laser trimmed. Since capacitors are formed by separating two conductive layers with a thin dielectric, such as oxide for MOS capacitors, laser trimming typically short circuits a capacitor. Further, this method of trimming must be done prior to final packaging of an integrated circuit and is therefore subject to variation.